A fuel cell is a device in which a fuel and an oxidizing agent are continuously and independently supplied to the anode and cathode electrodes, respectively, to undertake electrochemical reactions by which chemical energy is converted directly into electrical energy and by-product of heat. Fuel Cells are therefore inherently clean and efficient and are uniquely able to address the issues of environmental degradation and energy security. They are also safe, quiet and very reliable. In the PEM fuel cells the electrolyte is a thin polymer membrane (such as Nafion™, polybenzimidazole (PBI)) that is permeable to protons, but does not conduct electrons, and the electrodes are typically made from carbon. Hydrogen flowing into the anode is split into hydrogen protons and electrons. The hydrogen protons permeate across the electrolyte to the cathode, while the electrons flow through an external circuit and provide power. Oxygen, commonly in the form of air, is supplied to the cathode and combines with the electrons arriving from external circuit and the hydrogen protons migrated from the anode to produce water and heat. These reactions at the electrodes are as follows:    Anode: 2H2=4H++4e−    Cathode: O2+4H++4e−=2H2O+Heat    Overall: 2H2+O2=2H2O+Electricity+Heat
PEM fuel cells using lithium-based membranes normally operate at a temperature of around 70-80° C., but may also operate at a temperature of about 100 to 200° C. if a high temperature membrane (such as polybenzimidazole (PBI)) is used. At these temperatures the electrochemical reactions would normally occur slowly so they need to be stimulated by a thin layer of catalysts containing such as platinum on each electrode. This electrode/electrolyte unit is called a membrane electrode assembly (MEA) and it is sandwiched between two flow field plates (or separator plates) to create a fuel cell. These plates contain grooves to channel the fuel to the electrodes and also conduct electrons out of the assembly. Generally, each cell produces around 0.6-0.8 volt, about enough power to run a light bulb. In order to generate a higher voltage a number of individual cells are combined in series to form a structure known as a fuel cell stack.
To operate the fuel cell stack efficiently, it is desirable to distribute the reactants uniformly across the active area of the cell, i.e. the fluid flow field. These objectives are achieved by providing open-faced fluid flow plates (or called separator plates sometimes). The flow field plate generally serves essentially important multiple functions, including as current collectors to provide electrical continuity between the fuel cell voltage terminals and electrodes, and as mechanical support for the membrane electrode assembly (MEA) as well as distributing the reactants and water across the active area of the MEA. It is well known that the performance of fuel cell is highly dependent on the efficient transport and uniform distribution of the reactants to the electrode catalysts, and on the appropriate water management of the cell, i.e. the supply and removal of water produced during operation of the fuel cell. Flow field design affects the fuel cell performance by controlling the reactant concentration gradient, flow rate, pressure drop, water distribution, and current density profile as well as electrode catalyst utilization efficiency.
A variety of flow field designs exist in the art, with conventional designs generally comprising either pin type or serpentine type designs. An earlier example of the pin-type flow field design is illustrated in U.S. Pat. No. 4,769,297 issued to Reiser et al. published on Sep. 6, 1988 in which an anode flow field plate and a cathode flow field plate have each projections, which may be referred to as pins. The reactants (fuel or oxidant) flow across the flow field plate through intervening grooves formed by the projections. A similar design disclosed in U.S. Pat. No. 4,826,742 issued to Reiser published on May 2, 1989 having a pin-type design teaches a plurality of channels connected to an inlet and an outlet headers at the two ends. The headers extend to an opening that forms an inlet manifold or outlet manifold, respectively. The plate was designed for dead-ended operation with predetermined purge frequency. U.S. Pat. No. 6,403,249 B1 issued to Reid on Jun. 11, 2002 disclosed a flow field plate of typical pin-type design to apply for air-air moisture exchange of a PEM fuel cell.
Similar to the pin-type configuration, the flow field can be formed based on thin metal or carbon foils or wire mesh configurations, which may be simple diagonal path-equivalent patterns formed by various metals including stainless steels. Examples of this type are illustrated in U.S. Pat. Nos. 6,207,310 and 6,037,072 issued to Wilson and Zawodzinski on Mar. 27, 2001.
As addressed by Mohamed et al. in WO 02/069426 A2 published on Sep. 6, 2002, the pin-type flow field design features low-pressure drop across the corresponding flow field, a significant advantage resulting in low parasitic power consumption in relation to the reactants compression and delivery. However, the disadvantages of such flow field design may include: reactants channeling and formation of stagnant areas, as well as poor water management because the reactants flowing through flow fields always tend to follow the path of least resistance.
To date, most flow field designs have been of the so-called serpentine type. An example of a flow field having a single serpentine design is illustrated in U.S. Pat. No. 4,988,583, issued to Watkins et al., in which a single continuous fluid flow channel is formed in a major area of flow field plate. Another example of a single serpentine design is illustrated in U.S. Pat. Nos. 5,527,363 and 5,521,018 issued to Wilkinson et al. on Jun. 18, 1996 and May 28, 1996 and in U.S. Pat. No. 5,108,849 issued to Watkins on Apr. 28, 1992. A reactant enters serpentine flow channel through the inlet fluid manifold and exits through the fluid outlet to the outlet manifold after flowing over a major part of the plate. In a single serpentine flow channel the reactants are forced to traverse the entire active area of the corresponding electrode, therefore eliminating the formation of stagnant areas. However, the reactants flowing through a single long channel would obviously create a substantially high-pressure drop, which in turn requires large parasitic power consumption, and a significant reactant concentration gradient from the inlet to outlet would result in higher cell voltage loss. Furthermore, the use of a single channel may promote water flooding, especially at high current densities. This will also lower the cell performance and shorten the cell lifetime.
A number of patents have addressed on the high-pressure drop problem associated with a single serpentine design by providing multiple serpentine designs. In such designs reactants from the inlet manifold are directed into several continuous snacking flow channels to the outlet manifold. Examples of such multiple serpentine flow field designs are illustrated in U.S. Pat. No. 5,108,849 issued to Watkins on Apr. 28, 1992 in which continuous open-faced fluid flow channels traverse the plate surface in multiple passes, i.e. in a serpentine manner. Each channel has a fluid inlet at one end and a fluid outlet at the other end, i.e. the fluid flow in a channel is in a continuous manner. The fluid inlet and outlet of each channel are directly connected to the fluid supply opening (or inlet manifold) and fluid exhaust opening (or outlet manifold), respectively. Other fluid flow field plates having multiple serpentine designs are also disclosed in U.S. Pat. No. 5,108,849 issued to Watkins et al. on Apr. 28, 1992, U.S. Pat. No. 6,150,049 issued to Nelson et al. on Nov. 21, 2000, U.S. Pat. No. 6,500,579 B1 issued to Hideo et al. on Dec. 31, 2002, WO 02093672 A2 and WO 02093668 A1 issued to Frank et al. on Nov. 21, 2002.
U.S. Pat. No. 5,686,199 issued to Cavalca et al. on Nov. 11, 1997 disclosed a flow field plate design in which the plate is divided into a plurality of substantially symmetric flow sectors having separate inlets and outlets communicating with the networks of supply and exhaust flow passages, respectively, while each flow sector includes a plurality of substantially parallel open-faced flow channels with each sector partitioned so as to subdivide the channels into a plurality of sets of channels disposed in serial flow relationship. It is claimed that this configuration permits the reactant gases to be transported to the entire active area of the corresponding fuel cell electrode with relatively low reactant gas pressure drop. However, due to the different lengths of the network channels communicating with inlet and outlet manifolds it may lead to unequal pressure drop of gases to different flow sectors, and consequently the reactant gases may not be able to be distributed uniformly into these symmetric flow sectors. Furthermore, extra pressure drop may be caused because of change in flow areas of the flow channels when the gases flow from supply channel to the sector.
U.S. Pat. No. 6,099,984 issued to Rock on Aug. 8, 2000 disclosed a PEM fuel cell having serpentine flow channels wherein the gas manifold fluidly connects to a plurality of fluid inlet/input legs at one end and a plurality of fluid outlet/output legs fluidly connects to outlet gas manifold. The inlet legs of each channel border the inlet legs of the next adjacent channels in the same flow field, and the outlet legs of each channel border the outlet legs of the next adjacent channels in the same flow field. Each flow channel travels in a portion of the flow field in a serpentine manner. It may be understood that the same flow rates from manifold to the different inlets may be hard to achieve.
U.S. Pat. No. 6,309,773 B1 issued to Rock on Oct. 30, 2001 further disclosed a PEM fuel cell having serpentine flow field channels comprising a plurality of serially linked serpentine segments extending between inlet and outlet manifolds. Each segment has an inlet leg, an exit leg, at least one medial leg there between and hairpin curved ends connecting the medial legs to other legs of the segment. A bridging section of each flow channel connects adjacent segments of the same channel to the next. The hairpin curved ends of the medial legs are spaced from bridging sections by different distances depending on the difference in pressure in the bridging section and the hairpin curved ends. Compared to the typical serpentine flow field designs, this design seems more complex, and may lead to an even higher pressure drop.
U.S. Pat. Nos. 5,521,018 and 5,300,370 issued to Wilkinson et al. published respectively on May 28, 1996 and Apr. 5, 1994 also disclosed a multiple serpentine design of fluid flow field comprising a continuous region and a discontinuous region. WO 0148843 A2 to Wilkinson et al. on Jul. 5, 2001 further disclosed a fuel cell plate with discrete fluid distribution feature. The employment of discontinuous and discrete flow channels is expected to improve fuel cell performance by enhancing mass transfer, but this is only appreciated when pressurized reactants sources are used.
It is understood that serpentine design of fluid flow field can promote reactant flow across the active area of the plate, and forces the movement of water through each channel to reduce water flooding. However, this type of flow field design has apparent drawbacks including: (a) long, narrow flow paths typically involving a plurality of turns leads to a large unfavorable pressure drop between the inlet and outlet, thus creating the need for pressurization of the reactant supplies, which translates to a significant parasitic power load which in turn reduces the amount of power otherwise available for delivery; (b) flooding of the electrode due to poor removal and accumulation of water, which reduces the efficiency and lifetime of the fuel cell. To promote the water removal from long channels it is a common practice that a larger reactant supply is used to maintain a sufficient high reactant speed across the channels. For instance, as high as 2.5-3.5 of air stoichiometry is commonly applied in PEM fuel cell operations. This obviously requires larger parasitic power consumption. Furthermore, the requirement of high reactant supply leads to small turn down ratio, greatly limiting the system operation flexibility; (c) long flow channels result in high gradient in reactant concentration from the inlet to outlet, which creates larger drop in over potential; (d) high reactant gradients also leads to non-uniform current density distribution and ineffective utilization of electrode catalysts that, in turn, results in the use of a larger stack size, increasing the cost of the system.
It has been well known in the field that a small fuel cell having only a few square centimeters of active area can produce a power density of about 1-2 W/cm2, while the figure becomes typically 0.1-0.3 W/cm2 when the fuel cell is increased to a few hundreds of square centimeters. This phenomenon is referred to as scale-up effect. It has been predicted that the current density and hydrogen concentration along the flow channel length has similar pattern that they decrease along the channel (Hirata et al., Journal of Power Sources, Vol. 83, pp. 41-49, 1999). Several recent publications including Wang et al. (Journal of Power Sources, Vol. 94, pp. 40-50, 2001), Neshai et al. (http://www.utc.scsu.edu/effects.htm, last visited on Aug. 5, 2003), Li et al. (Journal of Power Sources, Vol. 115, pp. 90-100, 2003), and Yong et al. (Journal of Power Sources, Vol. 118, pp. 193-199, 2003) carried out numerical and experimental studies of two-phase flow and current density distribution across serpentine flow channels. These studies showed that there is a dramatic variation or decrease of the local current density along the stream, and at the end of flow stream the electrochemical reaction is extremely weak. Typically, it was found that the local current density does not significantly vary over the first half of the membrane surface, but it sharply decreases over the second half of membrane surface. The results of oxygen distribution showed a similar trend, i.e. most of the inlet oxygen has been consumed at the half-length of the flow channel, leaving the channel outlet section nearly depleted of oxygen, leading to inefficient utilization of catalysts. In the entrance half-section, high reaction rate occurs accompanied by excessive hydrogen and oxygen consumptions. Due to depletion over the second half section, the air flow slows down, which consequently could lead to accumulation of liquid water decreasing PEMFC performance by creating high gas resistance (which is known as water flooding). From the result of Wang et al., it is found that at the average current density of 1.4 A/cm2 the local current density is 2.22 A/cm2 at the inlet, but only 0.86 A/cm2 near the outlet. At the midway of the flow channel the current density showed a sudden drop that marks the beginning of liquid water formation, suggesting a poor water removal from the channels given that the oxygen is nearly depleted. This phenomenon is to some extent due to the fact that the reactivity of fuel cell electrochemical reaction gradually declines from the hydrogen and oxygen inlets to the outlets, as these are consumed along the flow passages.
To address on the above problem, Japanese Patent No. 6267564 disclosed a fuel cell plate having such flow passage that at least any one of the depth or the width of the oxygen-containing gas delivering plate gradually decreases from an upstream flow passage region to a downstream flow passage region. Given the fact that the plate itself is fairly thick and the serpentine flow channels are lengthy, the above method would not be expected to be easy in terms of manufacturing and machining. In this context, U.S. Pat. No. 6,048,633 issued to Fuji et al. on Apr. 11, 2000 disclosed a fuel cell plate on which the number of grooves on the fuel inlet side is set to be larger than the number of grooves on the outlet side. The grooves are gradually merged with each other as they come to the outlet side (i.e. the number of grooves will be reduced half by half in a stepwise manner, e.g. 12-6-3). When the number of gas flow passage grooves is an odd number, the number of grooves will be decreased to a half number obtained by adding one (1) to the odd number of gas flow passage grooves. The ratio of the number of grooves on the inlet side to the number of grooves on the outlet side is set to correspond to the gas utilization factor.
The method disclosed in U.S. Pat. No. 6,048,633 enhances the uniformity of reactant gases over the active area, promotes water removal and improves gas diffusion, and consequently improves cell performance. However, the manner that the number of grooves decreases half by half inherently limits the design flexibility in selection of the flow passage numbers. For instance it is more convenient to have even numbers that can be easily decreased half by half in a stepwise manner. If we assume a gas consumption rate of 50% (corresponding to 70% H2 utilization), and according to the patent, if the outlet has 5 channels, then it is only possible to have two passages, 10-5, giving a total 15 channels, which may not be enough to cover the required active area. Or, if a total of 100 channels needed to cover the required active surface, the channel arrangement will be about 67-33 (half-by-half reduction). With too many flow channels at each passage, the gas flow rate will be too low to properly remove the produced water. On the other hand, in order to cover all the active area and starting with 5 channels at the outlet, the number of grooves could be arranged like: 5-10-15-30-60-120. It follows that the ratio of groove numbers of inlet to outlet is no longer corresponding to the gas utilization factor (i.e. 50%). Furthermore, the method sets the number of grooves on the inlet side and the number of grooves on the outside corresponding to the gas utilization factor, but ignores the fact that the number of grooves in intermediate passages, when decreased half by half, does not provide uniformity as local gas flow rate decreases, generally following an exponential rule. This suggests that the flow rate may have dramatic change from one passage to the other, creating local non-uniformity and pressure loss due to suddenly acceleration or deceleration. Furthermore, the disclosed method actually only ensures nearly constant gas flow rate, not reactant molecules per active area.
In conclusion, there is accordingly a need for a fuel cell stack that overcomes the above-mentioned disadvantages of the prior art.